1. Field of the Invention
The present invention relates to a spindle unit for a machine tool having a spindle rotatably supported in a housing, and more particularly relates to a spindle unit for a machine tool which is provided at a forward end of a spindle rotatably supported in a housing with a hydraulic tool holding system for allowing a tool to be held in the spindle without a tool holder.
2. Description of the Related Art
In a tool machine, two types of spindle units, which are different from each other in a way of mounting a tool on a spindle rotatably supported in a housing of the spindle unit, can be used. In one type, a tool holder with a tool held therein is mounted on the spindle in order to mount the tool therein. In the other type, what is called a holderless spindle type, a tool holder with a tool held therein is not mounted on the spindle but only a tool is directly mounted therein. The holderless type of spindle unit is often used in a machine tool especially for machining a workpiece of complicated shape, such as a rib groove of a mold, by means of a relatively narrow tool having a small cross section.
The holderless type of spindle unit includes one having a mechanical tool holding system capable of gripping the shank portion of the tool by a collet or other means located at a forward end of a draw bar of the spindle unit and one having a hydraulic tool holding system capable of holding the shank portion of the tool by deforming a tool receiving bore for receiving the tool therein, which is formed in the forward end of the spindle unit, by means of a pressure medium (working fluid) such as hydraulic oil.
It should be noted that the term xe2x80x9cforwardxe2x80x9d or xe2x80x9cfrontxe2x80x9d herein refers to a side holding the tool and machining the workpiece or the like and that the term xe2x80x9crearxe2x80x9d herein refers to a side opposed to the side machining the workpiece or the like.
A mechanical tool holding system is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 8-174364, which includes a spindle formed with a tool receiving bore at a forward end thereof and a draw bar extending through the spindle along the axis thereof. During attachment or detachment of a tool, the mechanical tool holding system grips and releases a shank portion of a tool inserted into the tool receiving bore by a collet located at the forward end of the draw bar.
A hydraulic tool holding system is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 62-15043, which includes a spindle formed with a tool receiving bore at a forward end thereof and a pressure chamber surrounding the tool receiving bore at a distance. The hydraulic tool holding system changes the pressure of a working fluid enclosed in the pressure chamber so that it enlarges or reduces the diameter of the tool receiving bore in cross section in order to attach or detach a tool inserted into the tool receiving bore to/from the spindle.
Nowadays, the machining of a workpiece increasingly requires a much higher machining accuracy. Thus, this requirement arouses concern about factors which can cause degradation of machining accuracy, such as misalignment between the rotational axis of the spindle and a center axis of the tool mounted in the spindle, a change in actual tool length caused by deviation of an axial tool hold position where the spindle holds the tool along the length thereof.
In the case of an NC (numerical control) machine tool using an automatic tool changer, the above conventional tool holding systems suffer from the following drawbacks resulting from deviation of the tool hold position in the mounting of the tool.
In the tool holding system provided with the collet arranged at the forward end of the draw bar, the draw bar draws the collet rearward to grip the tool and this movement of the collet moves the tool in its axial direction, thereby to change the tool hold position. This may result in deviation of the actual tool length.
On the other hand, in the hydraulic tool holding system, a hydraulic chuck is secured on the front end face of the spindle by means of suitable fasteners such as screws, as described, for example, in. Japanese Unexamined Patent Publication (Kokai) No. 62-15043. Such a hydraulic tool holding system may rotate eccentrically the tool with respect to the center axis thereof due to misalignment between the rotational axis of the spindle and the center axis of the chuck. Thus, such a hydraulic tool system can adversely affect machining accuracy as a result of eccentric rotation of the tool.
Further, in order to adjustably change the fluid pressure in the hydraulic chuck secured to the forward end of the spindle, a fluid pressure adjusting screw provided in the hydraulic chuck must be turned at the forward end of the spindle by a wrench or the like. However, since a table and a workpiece are disposed in an area surrounding the forward end of the spindle, there may be insufficient space for working comfortably in the area. Therefore, it is not easy to change or adjust the fluid pressure, and this may reduce operating efficiency. In addition, when the automatic tool changer is used with the machine tool, the mechanical tool holding system is often used in combination therewith, because the operation of changing the fluid pressure must be performed at the forward end of the spindle in the conventional hydraulic tool holding system as described above. However, use of the hydraulic tool holding system makes it difficult to fully automate a tool change operation.
It is therefore an object of the invention to provide a spindle unit for a machine tool having a spindle and a holderless type of tool holding system, which can hold a tool so that the rotational axis of the spindle is in alignment with the center axis of the tool when the tool is mounted on the spindle, with substantially no deviation in the axial position of a tool held by the tool holding system, and facilitates automatization of the operations of holding and releasing tools.
The above object is accomplished by a spindle unit for a machine tool according to the present invention which is configured to form a tool receiving bore either directly in the forward end of the spindle or in an adapter fixedly secured to the forward end of the spindle so that the center axis of the tool receiving bore is in alignment with the rotational axis of the spindle, and to employ a hydraulic tool holding system which allows the fluid pressure of the hydraulic tool holding system to be adjustably changed by an operation performed on the rear side of the spindle, where it is easier to secure sufficient space for the operation to be comfortably performed.
In accordance with a first aspect of the present invention, there is provided a spindle unit for a machine tool having a spindle housing, which includes a spindle having a rotational axis and rotatably supported in the spindle housing; a tool receiving bore formed in the forward end portion of the spindle and extending along the rotational axis of the spindle; one or more pressure chambers disposed around the tool receiving bore in spaced relationship therefrom by a thin wall elastically deformable in a radial direction of the spindle, the pressure chamber being filled with working fluid; a working fluid passage formed in the spindle and in fluid communication with the one or more pressure chambers; and a pressure adjusting device arranged in the rear portion of the spindle unit and connected to the pressure chamber through the working fluid passage, the pressure adjusting device adjustably changing the pressure of the working fluid within the pressure chamber so that the pressure adjusting device can elastically deform the thin wall to enlarge or reduce the tool receiving bore in radius, thereby holding or releasing the tool inserted into the tool receiving bore.
In the aforementioned spindle unit, the spindle may include an adaptor mounted on the forward end of the spindle and the tool bore is formed in the adaptor.
In one embodiment of the spindle unit, a pressure element insert hole is formed in the rear end portion of the spindle to be in fluid communication with the working fluid passage, and the pressure adjusting device includes a pressure element inserted into the pressure element insert hole for pressurizing the working fluid in the working fluid passage and a pressure element driving device for driving the pressure element.
In this embodiment, preferably, the pressure element is a piston, and a space defined by the pressure element insert hole and the piston is filled with the working fluid and in fluid communication with the working fluid passage.
The pressure element may further include a pressure screw coupled to the piston, and the pressure element insert hole may be formed with a threaded portion for engaging with the pressure screw of the pressure element.
In this case, preferably, the pressure element driving device includes a motor having a rotatable wrench for rotating the pressure screw, an actuator for moving the wrench to engage the wrench with and disengage it from the pressure screw, and a controller for controlling the operations of the actuator and the motor. The motor may be a servomotor. More preferably, during a process for holding the tool in the spindle, the controller operates the servomotor in a first operating mode in which rotational speed control and position control are performed, and subsequently operates the servomotor in a second operating mode in which torque control is performed.
The pressure element driving device may be rotatably coupled to the piston. In this case, preferably, a compression spring is placed within the space so as to urge the piston in a direction to increase the pressure of the working fluid in the space.
A compression spring may placed within the space so as to urge the piston in a direction to increase the pressure of the working fluid in the space, and the pressure element driving device may be separate from the piston and move the piston against an urging force of the compression spring to release the tool in the tool receiving bore.
In another embodiment of the spindle unit, a check valve is disposed at the rear end of the working fluid passage opening at the rear end of the spindle, and the pressure adjusting device includes a movable working fluid feeding device able to be seated on the rear end face of the spindle, the working fluid feeding device opening the check valve to provide fluid communication between the working fluid passage and the pressure adjusting device when the working fluid feeding device is seated on the rear end face, thereby allowing the working fluid feeding device to control the pressure of the working fluid within the pressure chamber.
In accordance with a second aspect of the present invention, there is provided a spindle unit for a machine tool having a spindle housing, which includes a spindle having a rotational axis and rotatably supported in the spindle housing; an adaptor having a center axis and attached to the forward end of the spindle in a manner to align the center axis of the adaptor with the rotational axis of the spindle; a tool receiving bore formed in the adaptor and extending along the center axis of the adaptor; one or more pressure chambers disposed around the tool receiving bore in spaced relationship therefrom by a thin wall elastically deformable in a radial direction of the adaptor, the pressure chamber filled with working fluid; a working fluid passage formed in the spindle and the adaptor and in fluid communication with the one or more pressure chambers; and a pressure adjusting device arranged in the rear portion of the spindle unit and connected to the pressure chamber through the working fluid passage, the pressure adjusting device adjustably changing pressure of the working fluid within the pressure chamber so that the pressure adjusting device can elastically deform the thin wall to enlarge or reduce the tool receiving bore in radius, thereby holding or releasing the tool inserted into the tool receiving bore.
In accordance with a third aspect of the present invention, there is provided a spindle unit for a machine tool having a spindle housing, which includes a spindle having a rotational axis and rotatably supported in the spindle housing; a tool receiving bore formed in the forward end portion of the spindle and extending along the rotational axis of the spindle; one or more pressure chambers disposed around the tool receiving bore in spaced relationship therefrom by a thin wall elastically deformable in a radial direction of the spindle, the pressure chamber being filled with working fluid; a working fluid passage formed in the spindle and in fluid communication with the one or more pressure chambers; and a pressure adjusting device connected to the pressure chamber through the working fluid passage, the pressure adjusting device adjustably changing the pressure of the working fluid within the pressure chamber so that the pressure adjusting device can elastically deform the thin wall to enlarge or reduce the tool receiving bore in radius, thereby holding or releasing the tool inserted into the tool receiving bore.
In the spindle unit according to the present invention, the tool receiving bore extending along the rotational axis of the spindle is formed in the forward end portion of the spindle of the machine tool and at the same time the pressure chamber is disposed around the tool receiving bore. This arrangement allows the tool to be held in or released from the forward end of the spindle by adjustably changing the fluid pressure within the pressure chamber. Therefore, the spindle unit can eliminate or reduce deviation of the axial position of the tool held by the spindle, deviation being one drawback of the conventional tool holding system using a collet. The same effect can be achieved by the alternative spindle unit in which the adaptor preformed with a tool receiving bore, one or more pressure chambers and a working fluid passage is fixedly secured to the spindle so as to form a single unit.
Further, since changes of pressure of the working fluid within the pressure chamber are performed by a pressure adjusting device disposed in the rear portion of the spindle unit, the pressure change operations for holding the tool in or releasing it from the spindle do not have to be performed at the forward end portion of the spindle, in which the spindle, the housing for supporting the spindle and other parts may interfere with the operation due to lack of sufficient space for comfortable operation. This facilitates the operation of holding the tool in or releasing it from the spindle.
The arrangement of the pressure adjusting device at the rear portion of the spindle unit also facilitates automation of the operation for holding the tool in or releasing it from the spindle. For example, the tool holding or releasing operation can be automated by the pressure element being inserted into the pressure element insert hole opening to the rear end face of the spindle and by the pressure element driving device driving the pressure element to pressurize the working fluid within the working fluid passage.
Further, the tool holding or releasing operation can be automated by the check valve being arranged at the end of the working fluid passage opening to the rear end face of the spindle so that the movable pressure adjusting device can open the check valve when it is seated on the rear end face of the spindle and by the pressure adjusting device adjustably changing the pressure of the working fluid within the working fluid passage to change the pressure of the working fluid within the pressure chamber.
Thus, automated operation of holding the tool in or releasing it from the spindle can be achieved by the spindle unit, despite the use of the hydraulic tool hold system. Further, a completely automated tool change operation can be achieved when the spindle unit is used in combination with the automatic tool changer.
In addition, the tool receiving bore formed in the forward end of the spindle can eliminate the need to provide an adaptor with the tool receiving bore preformed therein. As a result, the tool receiving bore formed in the spindle prevents misalignment between the rotational axis of the spindle and the center axis of the tool receiving bore during mounting of the adaptor on the spindle and therefore makes it possible to hold the tool in the proper position.